C1-inhibitor is a plasma proteinase inhibitor of the serpin family. The physiological targets for C1-inhibitor are the activated C1r and C1s subunits of complement component C1, and enzymes of the contact system. Deficiency of C1-inhibitor, either hereditary or acquired can cause angioedema, a potentially fatal condition. Although C1-inhibitor has much in common with other serpins, it has some unique characteristics which have not been investigated in detail. Firstly, it inhibits proteinases slowly compared to other serpins. The structural reasons for this are not known. Initially it will be confirmed that C1s inhibition occurs slowly when in its physiological form, as part of C1. Then the structural basis for the inhibitory activity will be studied by use of C1-inhibitor mutants and C1-inhibitor- alpha1-proteinase inhibitor chimeras. C1-inhibitor has a highly glycosylated 110 residue amino-terminal domain which has no homology with other serpins. The function of this region will be investigated, in particular testing the idea that it is essential for rapid inhibition and subsequent dissociation of C1. To test this recombinant C1-inhibitor variants lacking part or all of the amino-terminal will be made. Also single cysteines will be introduced into the amino-terminal domain which can be labeled with fluorophore to assess the interaction of this region with C1 and the target proteinases. These topics will be studied in the following three aims: Aim 1) To determine the rate of C1s inhibition when isolated and when in the C1 complex; Aim 2) To determine the structural basis for the inhibitory activity of C1-inhibitor; and Aim 3) To determine the function of the amino-terminal domain of C1-inhibitor. These studies will provide important information regarding structure-function relationships in C1-inhibitor, which has implications for the in vivo function of the protein, the therapeutic use of the protein, and for the deficiency states caused by naturally occurring dysfunctional variants.